It’s a clash of the not-quite-titans when it comes to smallest mammal, as we have two miniscule insectivores in the running. The Bumblebee Bat (Craseonycteristhonglongyai), also called Kitti’s hog-nosed bat, is often cited as being the smallest mammal in terms of length and skull size. These bats are considered vulnerable by IUCN standards. They live in limestone caves along rivers in western Thailand and Burma, and may be at risk due to habitat loss.

Shrews in general offer stiff competition for the title of smallest mammal, and the white-toothed pygmy shrew (Suncusetruscus, also known as the Etruscan shrew) is the primary contender. This species occurs in Mediterranean lowlands from Portugal to the Middle East, wherein they favor forest, shrub, and grassland habitats. It may be a little lighter than the bumblebee bat on average, but reports vary. Animal Diversity Web puts the average weight of white-toothed pygmy shrews at 1.8-3 grams.

A pygmy shrew in the UK

I’m afraid there’s no clear winner here. Even the most reliable sources use phrases like “may be the world’s smallest mammal” to describe these two species. For perspective, both are, literally, about as big as a robust bumblebee. Neither of them is likely to be as long or as heavy as your thumb.

SMALLEST BIRD: Bee Hummingbird

It seems bees are an excellent size analogue when naming tiny animals. The bee hummingbird (Mellisugahelenae) actually holds two titles: smallest bird and potentially the smallest warm-blooded vertebrate. Again, they are aptly named, being about the size of a large bee, with males weighing 1.95 grams and females 2.6 grams, on average. Their average wingspan is a whopping 3.25 cm – approximately 1.28 inches. These birds live across the island of Cuba and Isla de la Juventud, a nearby island. Remarkably, though small animals tend to live short lives (the aforementioned shrew typically lives around 15 months), these birds have been known to live 7 years in the wild – and a full decade in captivity.

A video of a bee hummingbird.

SMALLEST REPTILE: JaraguaSphaero vs. Virgin Gorda Least Gecko

It’s another double-header for smallest reptile, with two geckos of the genus Sphaerodactylus sharing the title. Both lizards are approximately 16 mm (3/4 inches) from snout to vent (i.e. the base of their tail). These geckos can curl comfortably onto a dime. The Jaragua Sphaero (Sphaerodactylusariasae) was discovered on an island off the coast of the Dominican Republic in 2001. It is endangered by habitat destruction. S. ariasae shares the record with S. parthenopion, which was discovered in 1965 in the British Virgin Islands, and is of a comparable size.

SMALLEST AMPHIBIAN: Assorted Tiny Frogs

Our least clear-cut taxonomic category yet. There are several species of frog with snout-vent lengths of around 10 mm, which means would fit comfortably onto your fingernail. The most recently discovered is Noble’s pygmy frog (Noblellapygmaea), a tiny species found in 2009 at extreme elevations in the S. American Andes. What’s more, this species lays massive eggs which skip the tadpole phase and hatch as miniature adults. As far as the competition goes, according to AmphibiaWeb, “the smallest frog is probably from the genus Eleutherodactylus,” but other notably small frogs include “Psyllophrynedidactyla from Brazil and Stumpffia (S. tridactyla for example) from Madagascar.”

Several types of frog are very, very small, making a ‘smallest species’ declaration tricky.

SMALLEST INSECT: Fairyflies (family Mymaridae)

The final member of our countdown is the smallest. Fairyflies are not actually flies, but tiny wasps. Given the sheer diversity of insects on the planet, and the number that remain undiscovered or unclassified, any absolute claims about insect record-holders should be taken with a grain of salt. That said, the males of this family of parasitic wasps may be less than 0.14 mm in length. The males are actually SMALLER than some single-celled paramecium.

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Closing out this answer, let’s take a moment to remember just how utterly small most of the life on our planet actually is. These are some of the smallest animals on the planet, but most of the life on earth is tiny. In this list we glossed over bacteria and algae and many other tiny forms of life. They may not be as immediately interesting, but they are fundamentally important for life on earth.

That lists skips out on most groups of invertebrates, so I’d like to add a few.

Smallest arachnid – dust mite. The three species of this tiny animal are only 0.420 mm in length and 0.25-0.32 mm in width.

Smallest crustacean – daphnia. This planktonic animal can be as small as 0.2 mm in length.

Simpler animals include the smallest ones. For example, the rotifes are only 0.04 mm in length. Rotifers live a wide variety of moist and wet environments. Symbions make up another phylum. Depending on their life stage, they are either 0.084 mm long and 0.042 mm wide or reach a larger size of 0.347 mm x 0.113 mm. I’m sure there are plenty more of tiny animals. Flatworms can be 0.2 mm long, Bryozoa – 0.5 mm, Entoprocta – 0.1 mm, Loricifera – 0.1 mm, and so on.

Nice work! I decided to gloss over most invertebrates because (1) they are incredibly diverse and (2) incredibly small. Glad to see somebody won’t let these tiny wonders slip through the cracks, though.

As far as primates go, the Marmoset is one of the world’s smallest. They grow only to 8 inches (including their 3 inch tail) and are very social creatures, marking their territories in the wild through scent and are found in small groups between 2 and 6 members.